Generally electrical devices operate through electrical energy supplied by power modules. Many electrical devices demand higher power, such as virtual reality devices or laptops. However, a single high power module could not be carried about because of rules in different districts. Therefore, a plurality of low power module and connected in parallel could be used as a single high power module to supply high power and could be carried about across districts. However, error values exist between the converted voltages of power modules when those power modules are connected in parallel to supply electrical energy. The error values cause the decrease of voltage conversion efficiency and the increase of the system temperature, and then the power modules' efficiency becomes poor.